______,Drain.,Cook onion gently in 1/4 cup butter until lightly brown.,Stir in flour, salt and mustard.,Stir in milk until smooth.,Add pepper, celery salt and cheese; stir until cheese is melted.,Place carrots in a shallow casserole.,Pour cheese sauce over carrots.,Top with bread cubes which have been mixed with remaining butter.,Bake at 350° for 35 to 40 minutes.,Serves 6 to 8.